This invention relates in general to particle accelerators and especially to cyclotrons for accelerating heavy ions. More particularly, the invention relates to an isochronous cyclotron in which superconducting coils provide a radially-decreasing magnetic field to focus the ions at non-relativistic velocities and a radially-increasing field to focus the ions at relativistic velocities.
The conventional cyclotron comprises spaced apart coaxial magnetic pole pieces and a dee electrode structure disposed therebetween. The dee structure is energized to provide an alternating electric field at right angles to the magnetic field whereby charged particles introduced near the center of the system are accelerated. Because of the magnetic field, the particles describe a curvilinear orbit which expands as the particles gain energy by repeated passage through the electric field. The fundamental cyclotron principle requires that the time for one revolution of a particle in the magnetic field be constant and independent of particle energy. Thus the magnetic field, the alternating electric field, and the frequency thereof, may be held at fixed values throughout the period of particle acceleration. However, because of two factors serious disadvantages occur in conventional cyclotrons when it is desired to accelerate particles to extremely high energies. First, the maximum magnetic-field strength supplied by the iron pole pieces is limited to approximately 2.2 K gauss. Consequently, high-energy ions have large-radius orbits which require pole pieces of great size and weight. The second problem is related to the increase in mass experienced by the particles as they are accelerated into the relativistic range of velocities and to the effect this phenomena has on the orbital frequency and orbital stability of particles. At non-relativistic velocities particles may be accelerated in stable, isochronous orbits by a magnetic field which decreases slowly in intensity from the center of the system outward. As the particles are further accelerated to relativistic velocities in a radially-decreasing field, the relativistic mass increase reduces the particles velocity, thereby changing the orbital frequency of the particle and causing the particles to arrive at the accelerating gaps out of phase with the accelerating electric field. One means of compensating for this effect is to vary the frequency of the alternating electric field as a given pulse of particles is accelerated. Althrough a satisfactory phase relationship is established, this technique severely restricts the total beam current.
A second means of compensating for the relativistic mass increase of the particles is to provide a magnetic field which increases in intensity as the particles move outward from the center of the system. However, in the past the radially-increasing fields have adversely affected the stability of the particle orbits.